1. Field of Invention
This invention relates generally to communications, and more particularly, to a method and apparatus for optimizing channel capacity in bundled multi-line digital subscriber line communications.
2. Description of the Related Art
North American Integrated Service Digital Network (ISDN) Standard, defined by the American National Standard Institute (ANSI), regulates the protocol of information transmissions over telephone lines. High data rates may easily be achieved over the trunk lines, which connect the telephone companies' central offices. The problem lies in passing these signals across the subscriber line between the central office and the business or residential user. These lines were originally constructed to handle voice traffic in the narrow band between 300 Hz to 3000 Hz at bandwidths equivalent to several kilo baud.
Digital Subscriber Lines (DSL) technology and improvements thereon including: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc. all of which are broadly identified as XDSL have been developed to increase the effective bandwidth of existing subscriber line connections, without requiring the installation of new fiber optic cable. An XDSL modem operates at frequencies higher than the voiceband frequencies, thus an XDSL modem may operate simultaneously with a voiceband modem or a telephone conversation.
Currently there are over ten discrete xDSL standards, including: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc. Within each standard there are several line codes or modulation protocols the most popular of which is discrete multi-tone (DMT). DMT modulation involves establishing a communication channel with a plurality of sub-channels each with a center frequency a.k.a. carrier tone. The sub-channels are frequency division multiplexed across the available bandwidth. Each sub-channel uses quadrature amplitude modulation (QAM) to modulate information. A typical DMT system utilizes a transmitter inverse fast Fourier transform (IFFT) and a receiver fast Fourier transform (FFT). XDSL modems are typically installed in pairs, with one of the modems installed in a home and the other in the telephone companies central office (CO) switching office servicing that home. This provides a direct dedicated connection to the home from a line card at the central office on which the modem is implemented through the subscriber line or local loop.
Any digital communication experiences signal interference, and communication protocols which support multiple sub-channels such as DMT are no exception. Interference can effect both the amplitude and the phase of the sub-channels. Such noise can arise across the time and/or frequency domains. At the receiver the data has to be separated from the noise. The current solutions to interference include individual and aggregate subscriber line approaches. Individual subscriber line approaches include: various forms of forward error correction (FEC) which add additional overhead to the data transmitted; carefully regulated band plans which frequency division multiplex upstream and downstream communications into discrete and non-overlapping frequency regions and capped transmission levels, a.k.a. power spectral densities (PSD) masks which limit interference between subscriber lines in a bundle by uniformly reducing bandwidth on all channels irregardless of the actual spectral profile of the channel. Aggregate subscriber line approaches involve vector cancellation applied to all the subscriber lines in a bundle with the training necessary to calculate vectors accomplished during the initialization of all lines.
With each improvement in bandwidth of multiple sub-channel communication systems there is a corresponding increase in noise, with the potential to reduce signal integrity to unacceptable levels. What is needed is a method and apparatus for increasing signal integrity in digital communication systems which support multiple sub-channels.